#include <iostream>

double cifre(FILE*);

int main()
{
	FILE* file2 = fopen("cifre.in", "r");
	double n = cifre(file2);
	fclose(file2);

	return 0;
}

double cifre(FILE* file)
{
	int a = 0;
	double b = 0;
	int c = 0;
	int k = 0;
	int count1 = 0;
	double count2 = 0;
	int clona, rest;
	double prob = 0;
	fscanf(file, "%d", &a);
	fscanf(file, "%lf", &b);
	fscanf(file, "%d", &c);
	fscanf(file, "%d", &k);

	clona = a;
	while (a <= b)
	{
		while (clona != 0)
		{
			rest = clona % 10;
			clona /= 10;
			if (rest == c)
			{
				++count1;
			}
		}
		if (count1 >= k)
		{
			++count2;
		}
		++a;
		clona = a;
		count1 = 0;
	}

	prob = count2 / b;
	return prob;
}